1. Field
This disclosure relates to an electrolyte for a rechargeable lithium battery including an additive and a rechargeable lithium battery including the same.
2. Description of the Related Art
Lithium rechargeable batteries have recently drawn attention as a power source of small portable electronic devices. They use an organic electrolyte solution and thereby have twice the discharge voltage of a conventional battery using an alkali aqueous solution, and accordingly have high energy density.
As for negative active materials of a rechargeable lithium battery, various carbon-based materials such as artificial graphite, natural graphite, and hard carbon, which can all intercalate and deintercalate lithium ions, have been used.
For positive active materials of a rechargeable lithium battery, chalcogenide compounds that are composite metal oxides such as LiCoO2, LiMn2O4, LiNiO2, LiNi1−xCoxO2 (O<x<1), LiMnO2, and the like have been researched.
For an electrolyte, a lithium salt dissolved in a non-aqueous solvent including ethylene carbonate, dimethyl carbonate, diethyl carbonate, and the like has been used.
During the initial charge of a rechargeable lithium battery, lithium ions, which are released from the lithium-transition metal oxide positive electrode of the battery, are transferred to a carbon negative electrode where the ions are intercalated into the carbon. Because of its high reactivity, lithium reacts with the carbon negative electrode to produce Li2CO3, LiO, LiOH, etc., thereby forming a thin film on the surface of the negative electrode. This film is referred to as an organic solid electrolyte interface (SEI) film. The organic SEI film formed during the initial charge not only blocks (or prevents) the reaction between lithium ions and the carbon negative electrode or other materials during charging and discharging, but it also acts as an ion tunnel, allowing the passage of only lithium ions. The ion tunnel blocks (or prevents) the disintegration of the structure of the carbon negative electrode, which causes organic solvents in an electrolyte with a high molecular weight to make solvate lithium ions, and the solvent and the solvated lithium ions co-intercalate into the carbon negative electrode. Once the organic SEI film is formed, lithium ions do not again react with the carbon electrode or other materials such that an amount of lithium ions is reversibly maintained.
However, problems may occur in which gases are generated inside the battery using carbonate-based organic solvents due to decomposition of a carbonate based organic solvent during the organic SEI film-forming reaction. These gases include H2, CO, CO2, CH4, C2H6, C3H8, C3H6, etc. depending on the type of non-aqueous solvent and negative active material used. Due to gas generation inside the battery, battery may be expanded during charge. In addition, a SEI film is slowly disintegrated by electrochemical energy and heat energy, which increase with the passage of time when the fully charged battery is stored at high temperatures after it is charged, for example, if it is stored at 85° C. for four days after a 100% charge at 4.2 V. Accordingly, a side reaction in which an exposed surface of the negative electrode reacts with surrounding electrolytes occurs continuously to generate gases. The internal pressure of the battery increases with this generation of gases.
Therefore, there is a need to develop an electrolyte additive to suppress an internal pressure increase by preventing or reducing this generation of gases during SEI film-forming reactions, and also to improve capacity retention at a high temperature.